Serpentinites form when seawater reacts with peridotite - rocks
that form the Earth's mantle and have been brought up to the seafloor
by tectonic processes. This picture shows a hand sample of a serpentinite
recovered from the Atlantis Massif. Thin fractures in the serpentinite
are filled with calcium carbonate. Sample is 16cm wide. Click image for larger view and image credit. (HR)

The The Lost City 2005 Expedition

What Drives Hydrothermal Activity at Lost City?

The chemistry of the vent fluids and fluid circulation at Lost City are
not driven by interaction of seawater with hot lava or by cooling of magma
at depth. Instead hydrothermal activity at Lost City is driven by chemical
reactions between seawater and mantle rocks that make up the underlying
basement. This is unlike almost all other known hot spring systems on the
seafloor.

The mantle, or peridotite, rocks that comprise the Atlantis massif generally
occur at much greater depths in the Earth - typically at depths greater
than about 6 kilometers. Because of faulting these mantle rocks are now
exposed at or near the seafloor and they are out of equilibrium with their
surroundings. These rocks contain large amounts of the mineral olivine
(a Mg-Fe silicate) which reacts with seawater at temperatures below 400°C
and forms serpentine minerals (hydrous Mg-silicates) and magnetite, an
iron oxide that is highly magnetic. This process, referred
to as serpentinization, has major geophysical, geochemical and biological
consequences for the marine system. It is a fundamental process at slow
spreading ridge environments where mantle rocks are commonly exposed near
the seafloor.

This photomicrograph shows a serpentinite in a polarizing
petrographic microscope. During reaction with seawater serpentine (the
grey, "snake-like" minerals)
forms what is called a mesh texture around olivine grains. The light
colored, round minerals are relict grains of olivine. Field of view
is 2.5 mm. Click image for larger view and image credit. (HR)

Serpentinization: The Heat Engine at Lost City and Sponge of the Oceanic
Crust

An important consequence of serpentinization is the production of heat.
The process of serpentinization can provide heat to drive the Lost City
hydrothermal system in two ways. First, the mantle rocks underlying Lost
City have residual heat from the mantle that can be "mined" through
cooling with seawater. A second important source of heat is the alteration
reactions themselves, which are termed exothermic because they consume
water and produce a significant amount of heat during the transformation
of olivine to serpentine and magnetite. The amount of heat produced is
directly proportional to the amount of water that is taken up to form the
mineral serpentine. In fact, serpentinization consumes an average of about
300 kilograms (approximately 300 liters or 79 US gallons) of water per
cubic meter of rock that is altered. At the same time, this process produces
about 660,000,000 joules of heat per cubic meter of rock.

The geological significance of heat production is that serpentinization
processes are capable of raising the rock temperature by about 260°C
(550°F) when not including processes that lead to heat transport and
cooling of the rock. It is this heat source that helps drive the Lost City
hydrothermal system. This is in strong contrast to other known hydrothermal
systems along the mid-ocean ridges which are driven by magmatic heat and
are characterized by high temperature, metal-rich fluids that ultimately
create spectacular "Black Smoker" sulfide structures.

Face Lifting through Serpentinization

Another important consequence of the formation of serpentine during hydration
of mantle rocks is that the density of the rocks change from about 3.3
grams per cubic centimeter (g/cm3) to about 2.7 g/cm3 and the volume of
the rocks can increase by 20-40%. Serpentinization also affects other geophysical
properties of the oceanic crust by lowering the seismic velocities of the
rocks, changing their gravity signatures and mechanical properties, and
by increasing their degree of magnetism. The change in density and the
expansion of the rocks during serpentinization has the important effect
that the mountain becomes lighter and needs more room as it swells up,
and thus "lifts" itself to greater elevations. The expansion
of the rocks also leads to a greater number of cracks and ultimately causes
mass wasting along steep slopes. This in turn creates new fractures that
allow seawater to penetrate further into the mountain and react with new
portions of rock that contain fresh olivine. Thus, the access of seawater
to relatively cool fresh peridotite, controlled by the processes of faulting,
volumetric expansion and mass wasting, is crucial to sustain Lost City-type
systems.

Radiogenic carbon age dating of the carbonate structures and overlying
sediments indicates that the Lost City hydrothermal system has been ongoing
for at least 30,000 years. This is at least two orders of magnitude longer
that most of the known black smoker systems. Geophysical data also suggests
that there is a significant amount of fresh peridotite at depth in the
massif that can still be altered, and thus the serpentinization processes
in the basement rocks have the potential to drive the Lost City system
for hundreds of thousands, if not millions, of years.

Serpentinized peridotites are the most dominant rock type
recovered during past expositions on the south wall of the Atlantis
Massif and form the basement of the Lost City hydrothermal vent field.
This outcrop picture shows partially deformed rocks that occur at the
top of the massif. The arm of the 3-person submersible Alvin is shown
in the right portion of the image. Click image for larger view and image credit. (HR)

Creating a long-lived, gas-rich hydrothermal environment

Because the Atlantis Massif consist mostly of peridotite and
have a distinctly different composition than basalts and slower cooled
counterparts (i.e. gabbros), the fluids that circulate beneath Lost City
have significantly different compositions than black smoker systems. During
alteration of the mantle rocks beneath Lost City, the fluids become extremely
basic and reach a pH of 9 to 11. This high pH is very important because
when the Lost City fluids are expelled from the chimneys, they mix with
seawater and trigger chemical reactions that cause calcium carbonate (CaCO3)
and brucite [Mg(OH)2] to precipitate. As long as the Lost City fluids maintain
a high pH, abundant limestone will continue to be deposited and the chimneys
will continue to grow.

The formation of magnetite during the serpentinization process involves
the oxidation of ferrous iron (Fe2+) in olivine to form ferric iron (Fe3+)
in magnetite and leads to what is called reducing conditions. As a consequence,
reduced gas species, such as hydrogen gas (H2), methane (CH4) and hydrogen
sulfide (H2S), can be produced during serpentinization. These dissolved
gas species provide important energy sources for microbial activity at
Lost City. Thus, the basement rocks and the porous Lost City structures
that are bathed volatile-rich, highly alkaline fluids create vital niches
for life.

The gases produced at Lost City, the organisms that thrive in them and
the long-lived nature of this serpentinite-driven system are very distinct
from black-smoker hydrothermal vents and may represent a modern analog
for some of the oldest hydrothermal systems on Earth. Thus, understanding
this system may provide important new insights for studying early life
on Earth as well as for looking for signs of life on other planets.